The present invention relates to decellularized vascular prostheses that are resistant to thrombus occlusion and have a low level of immunogenicity. The vascular prostheses are denuded of cells, and coated with an anti-thrombogenic agent and a growth factor that promotes recellularization and further reduces the immunogenicity.
Chronic venous insufficiency is a major health problem in the United States and throughout the world. More than 7 million people are afflicted and at least 500,000 develop leg ulcerations as a consequence. An estimated 900,000 new cases arise annually. Chronic venous insufficiency is a general term that encompasses all causes of chronic venous disease. It occurs in a primary form with stretched valves and dilated venous walls, and in a secondary form following thrombophlebitis, with scarred and deformed valves and thickened venous walls with longitudinal septa and seriously compromised lumens. Other causes of venous insufficiency, such as valve aplasia, congenital malformations and external obstruction occur less often.
The clinical symptoms associated with venous insufficiency range from severe pain and recurrent ulcerations to no manifest symptoms. The site of involvement appears to be critical to the severity of the symptoms. Thus, varicosity of the superficial venous system is usually benign and the incidence of significant complications is low. In contrast, insufficiency of the deep veins or of the perforating vessels is more frequently associated with pain, swelling, ulceration, and long-term disability.
The current basic treatments for venous insufficiency rely on the prevention of reflux and a reduction of venous pressure. Conservative treatments, however, including bed rest, limb elevation, mild diuretic administration, and elastic compression stockings are aimed at the relief of symptoms rather than the underlying disease process. They are not particularly successful.
Direct valvuloplasty may be accomplished by tightening redundant cusp edges, whereas indirect valvuloplasty employs a DACRON or polytetrafluoroethylene (PTFE) cuff around the valve. Despite noticeable gains in hemodynamic measurements, clinical improvement is frequently less evident. Venous valve repair and replacement are attempts to restore competence to the deep venous system. Venous valve repair, however, suffers from the limitation that it is only suitable for those patients without prior deep venous thrombosis. In the event that the valve apparatus has been significantly degraded or destroyed, valve transplantation may be the only available option to offer symptomatic relief and a fall in venous pressure.
The quantity and quality of donor valves remain significant problems. In the typical patient as many as 30% to 40% of brachial or axillary valves are incompetent. Additionally, many patients have dilated venous systems that will not accommodate a smaller-caliber brachial or axillary vein graft. Accordingly, valve transplantation suffers from considerable constraints in its use as a surgical technique.
Small caliber vascular grafts with inner diameters of less than 6 mm are used extensively in aorta-coronary artery and infrapopliteal artery bypasses for the treatment of arterial occlusive diseases, and as arterio-venous conduits for hemodialysis access in the end stage of renal disease. At present, autogenous saphenous veins continue to be the most widely used vascular prostheses for small caliber arterial reconstructive procedures. Primary patency at four years for an arterial bypass with saphenous veins is 40-70%. A practical impediment to constructing such bypasses, however, is the fact that 10 to 40% of patients do not have an acceptable saphenous vein that can be transplanted for a successful graft.
Previous harvesting of vascular tissue for use in cardiac or vascular surgical procedures, varicose vein stripping, and prior thrombophlebitis are the most common reasons for unsuccessful autogenous saphenous vein grafting. Alternative sources of small-caliber vascular prostheses, with a patency rate comparable to or better than that of the autogenous saphenous vein, are urgently needed for clinical use.
Venous allografts from cadavers have also been used. They provide reasonable function early in the life of the graft, but yield poor results after 2 years. Modern cryopreservation techniques, including controlled-rate freezing, storage at xe2x88x92190xc2x0 C., and cryoprotectants such as dimethyl sulfoxide and chondroitin sulfate, improve the viability of cryopreserved allograft saphenous veins. Successful results using unmodified cryopreserved allograft saphenous veins for infrainguinal tibial artery reconstructions have achieved a one-year patency rate in the range of 10 to 50%. Long-term benefits to the patient have been marred, however, by vein graft rejection and unheralded early graft closure. Complications related to the mechanical failure of the conduit itself, such as graft aneurysms or ruptures, have occurred with greater frequency and caused greater morbidity, compared to fresh autogenous veins.
Synthetic DACRON and PTFE vascular prostheses have achieved some degree of clinical success even though they are not ideal in large and mid-sized arterial reconstructions. In addition, vessel substitutes smaller than 6 mm in diameter are susceptible to early graft occlusion. The most frequently encountered failures of synthetic grafts result from thrombosis and anastomotic hyperplasia. The inherent properties of synthetic graft materials, and their limited spontaneous re-endothelialization in humans, contribute to high surface thrombogenicity.
The implantation of glutaraldehyde-fixed bovine and human umbilical vein grafts was extensively evaluated and largely discarded because of high rates of aneurysm formation occurring two years after implantation. Most of these grafts failed because of delayed vascular healing and degenerative changes. An immune response to the highly immunogenic, chemically modified venous material, was characterized by invasion of multinucleated giant cells and reduced implant recellularization. Furthermore, glutaraldehyde fixation disturbed the natural matrix protein configuration. The cytotoxic effect of glutaraldehyde inhibited cell migration into the graft wall. Degeneration in the grafts resulted in a highly thrombogenic surface and the consequent occlusion of the vessels by thrombosis.
Many factors contribute to the degree of patency achieved with a particular prosthesis. These include the inherent properties of the chosen materials, surface thrombogenicity, compliance, and porosity in the case of textile grafts. The surface properties of materials seem to be a key issue in securing the desired long-term patency of small vessel substitutes. Numerous researchers have attempted to optimize the clinical efficacy of small diameter vascular grafts by modifying the prosthetic materials to make them biologically inert, but such an inert material has yet to be developed. An alternative approach to optimize the biological components of the prosthesis-tissue complex has led to the development of biohybrid materials. Some examples include synthetic material seeded with viable cells, coatings of biological compounds such as albumin and collagen, and materials synthesized from polymers known to elicit favorable biological responses. This approach has also not yielded a practical or effective vascular prosthesis.
In general, biological materials obtained from animals or humans have unique and special microstructural, mechanical, hemodynamical, and biochemical properties that cannot be completely replicated by currently available technology. Therefore, biologically-derived materials have great potential as raw materials for implantable artificial organs. The use of porcine organs for xenotransplantation is an attractive option to overcome the shortage of available organs for transplantation into humans. However, the problem of acute rejection remains an unsolved barrier. Cell surface molecules of xenogenic organs are mainly responsible for eliciting host rejection responses. Thus, immune rejection of allogenic or xenogenic tissues and the resulting decrease in long-term durability of the graft are major obstacles to the successful development of the ideal graft.
The most immunogenic portions of allogenic or xenogenic vascular grafts are the cellular components. Mature collagen, in contrast, shows low or no antigenicity, especially when transferred from individuals of the same species. For example, induction of an immunological response against purified bovine collagen is extremely low when injected for cosmetic purposes, or in vascular grafts impregnated with bovine collagen. Chemical cross-linking of collagen, on the other hand, renders the collagen highly immunogenic and can drastically reduce its biocompatibility with the host.
A readily available, synthetic, biologic or biohybrid venous valve in various sizes would greatly facilitate valve reconstruction surgery, including the desirable goal of valve insertions in multiple sites. The development of a vascular vessel or venous valve prosthesis that avoids the long-term likelihood of thrombosis and immune rejection would revolutionize the treatment of chronic venous insufficiency.
Problems of thrombogenicity and poor tissue compatability are also encountered with implantable vascular stents. Vascular stents are supporting devices, used to strengthen or dilate a blood vessel after balloon angioplasty or endarterectomy. They are made of synthetic material that is typically thrombogenic and has poor tissue compatibility. Stents fail primarily due to thrombotic occlusion and restenosis from tissue overgrowth.
What is needed, therefore, is a graft that is durable and can maintain structural integrity. Specifically, it must retain mechanical strength in all dimensions so that dilational and elongation distortions are minimized. The graft must be capable of long-term storage. It must be available in many sizes to accommodate the wide variation of vascular reconstructions. The prosthesis must resist infection. Intraoperatively, the ideal graft should have excellent handling characteristics, including flexibility, ease of suture placement, and minimal needle-hole and interstitial bleeding. The compliance of the ideal graft should closely approximate that of the host vessel. Ideally, turbulence about the anastomoses should be minimized to decrease intimal hyperplasia. In addition, the luminal surface should be resistant to platelet aggregation and thrombosis following placement in the patient, and avoid the immunogenicity that can arise from chemical modification of biological material. Finally, in this era of cost-containment, the graft must be relatively inexpensive and easy to manufacture.
The present invention solves the problems described above by providing decellularized vascular prostheses covalently linked with at least one anti-thrombogenic agent and at least one growth factor. The decellularized blood vessels and vascular valves comprise collagen and elastin matrix proteins of minimal immunogenicity, an anti-thrombogenic agent and a growth factor to promote recellularization of the graft.
The vascular prostheses that are the subject matter of the present application have novel properties that offer advantages over current synthetic or unmodified natural vascular grafts. Decellularization of the grafts eliminates the major factor inducing immunological rejection. These prostheses are also suitable substrates for host vascular cell invasion, cell attachment, proliferation, migration, and differentiation, since they consist of native matrix proteins.
Covalent linkage of the vascular prostheses with an anti-thrombogenic agent, including but not limited to heparin, offers a non-covalent attachment site for growth factors such as heparin-binding growth factors including, but not limited to, basic fibroblast growth factor (bFGF) that will improve and accelerate vascular healing and the remodeling process. Thus, the bioengineered biological vascular prostheses of the present invention have long term patency and less likelihood of post-operative complications. These grafts benefit patients by increasing the rate of recovery and decreasing the possibility of rejection or blockage of the graft.
The decellularized grafts of the present invention consist primarily of collagen and elastin matrix proteins, which are highly conserved among species and have low immunogenicity. This permits the use of xenogenic grafts from one species to another, reduces the current reliance upon allogenic sources of transplant material, and significantly expands the supply of available prostheses. The decellularized matrix is stable during long-term storage so that a graft of choice will be readily available when needed. This property allows the vascular surgeon to select a prosthesis that more closely matches the diameter of the recipient blood vessel. An improved blood flow is thereby obtained that is less likely to result in anastomotic thrombus formation.
The vascular prostheses of the present invention also permit other pharmaceutically active agents to be bound or otherwise immobilized to the immunologically inert biological matrix. This, and other advantages of the present invention, cannot be achieved by currently available technologies.
Accordingly, it is an object of the present invention to provide a vascular prosthesis that may include an anti-thrombogenic agent immobilized to the decellularized vascular prosthesis to create surfaces with reduced thrombogenicity.
It is another object of the present invention to provide a vascular prosthesis with reduced immunogenicity that retains a high degree of mechanical strength for long-term durability and suitability for surgical implantation.
It is yet another object of the present invention to provide a vascular prosthesis that is stable during storage.
Yet another object of the present invention is to provide a decellularized vascular prosthesis that retains sufficient mechanical strength to resist aneurysm formation, and supports surgical stitching with minimal leakage at the point of suture.
Another object of the present invention is to combine decellularized, anti-thrombogenic, growth factor-bound vascular prostheses with synthetic vascular stents and stent-valve devices.
Still another object of the present invention is to provide a method to decellularize vascular tissue so that it has reduced thrombogenicity and immunogenicity and approximates the mechanical strength of the native blood vessel.
It is also an object of the present invention to provide a method of linking decellularized vascular tissue with at least one anti-thrombogenic agent and applying a second linking of at least one cellular growth factor so that the modified vascular tissue may be used as a vascular prosthesis.
The decellularized, heparinized, and growth factor-bound vascular tissues of the present invention are superior to currently used prostheses since they provide xenogenic grafts with reduced propensity towards immunological rejection.
Still another advantage of the present invention is that the decellularized, heparinized, growth factor-bound vascular prostheses have application in multiple vascular replacement or reconstruction procedures, venous valve transplantations for chronic venous insufficiency, vein replacement, heart valve replacement, and as a vascular patch after carotid artery endarterectomy, femoral artery thrombectomy, and other vascular wall repairs.
A further advantage of the present invention is that the combination of a decellularized vascular implant, heparin, and the heparin-binding growth factor provides a new platform technology for development of many medical products.
These and other features, objects and advantages of the invention and preferred embodiments of the present invention will become apparent from the detailed description that follows.